Lignocellulosic materials, which are primarily comprised of cellulose, hemicellulose and lignin, are highly combustible. Such materials include solid lumber, laminated veneer lumber, parallel strand lumber, timber strands, plywood, medium density fiberboard (MDF), hardboard, particleboard, oriented strand board (OSB) and strawboard. For obvious reasons, fire retardant and/or fire resistant lignocellulosic materials are highly desirable.
In recognition of the foregoing, various attempts have been made to impart fire protection to wood based materials. For example, U.S. Pat. No. 4,514,326 discloses impregnating cellulosic materials with alkaline earth metal tetraborates, which are treated with ammonia, sulfuric acid, sulfur dioxide, or phosphoric acid, in order to provide such materials with some degree of flame retardancy. Common disadvantages associated with impregnating cellulosic products with such materials are the reduction in panel strength, service life and product reliability.
Heat expandable graphite and its use for providing flame resistance is well known in the art and described, for example, in U.S. Pat. No. 3,574,644, the disclosure of which is incorporated herein by reference. Heat expandable graphite is formed by treating crystalline graphite, which is composed of stacks of parallel planes of carbon atoms, with intercalants such as sulfuric acid and/or nitric acid. Since no covalent bonding exists between the planes of the carbon atoms, the intercalant can be inserted therebetween. This allows the intercalant to be positioned within the graphite lattice. When the intercalated graphite is exposed to heat or flame, the inserted molecules decompose and release gases. The graphite layer planes are forced apart by the gas and the graphite expands, thereby creating a low-density, non-burnable, thermal insulation that can reflect a high percentage of heat.
U.S. Pat. No. 5,443,894 discloses a fire retardant oriented strand board which includes expandable graphite containing outer layers. The outer layers contain resin bonded wood strands with up to 20% by weight of expandable graphite particles dispersed therein. Upon exposure to external flame, the expandable graphite particles undergo intumescence and increase in volume thereby forming a layer of vermiform expanded graphite. This layer creates a flame barrier for the underlying core layer. However, the use of expandable graphite in the outer layers has been found to cause a "popcorn effect" when such board is exposed to flame. This results in the outer layer fracturing and being blown off the panels. As the outer layer is blown off the panels, noxious gases and fumes, such as sulfur dioxide and nitrogen monoxide are given off.
Another approach for imparting fire resistance to cellulosic materials is disclosed in U.S. Pat. No. 5,434,200. This patent teaches combining finely divided lignocellulosic material with an amount of a magnesium oxychloride and/or magnesium oxysulphate containing latex effective to impart water and fire resistance to the lignocellulosic material. U.S. Pat. No. 5,246,652 discloses a method of producing a wood composite which includes treating a wood composite with a soluble boron compound to provide some degree of fire retardancy. A common drawback with each of these approaches is that high levels of noxious gases are created when the cellulosic materials are exposed to intensive heat and flames.